Mottle preventing treatment for paperboard

ABSTRACT

PAPER AND PAPERBOARD, FOR USE IN PACKAGING DETERGENTS CONTAINING A PERSALT SUCH AS SODIUM PERBORATE, ARE TREATED WITH A WATER-SOLUBLE INORGANIC ALKALI SALT SUCH AS SODIUM NITRITE AND A BUFFERING AGENT SELECTED FROM THE GROUP CONSISTING OF CALCIUM CARBONATE, MAGNESIUM CARBONATE AND SODIUM SILICATE, TO STABILIZE THE SODIUM NITRITE IN THE PAPERBOARD, AND PREVENT THE PAPER AND PAPERBOARD FROM BECOMING MOTTLED IN THE PRESENCE OF THE PERBORATE CONTAINING DETERGENT UNDER EXTREMELY ADVERSE CLIMATIC CONDITIONS DUE TO OXIDATIVE DEGRADATION OF CARBOHYDRATE AND PROTEIN MATERIALS IN AND ON THE PAPER OR PAPERBOARD.

United States Patent M 3,723,164 MOTTLE PREVENTING TREATMENT FORPAPERBOARD Joseph Marton and Terezia Marton, Silver Spring, Md.,assignors to Westvaco Corporation, New York, N.Y. No Drawing. Filed Apr.29, 1971, Ser. No. 138,809

Int. Cl. B44d l/14 U.S. Cl. 117-76 P 4 Claims ABSTRACT OF THE DISCLOSURESUMMARY OF INVENTION The present invention relates generally to thetreat-- ment of paperboard with materials to inhibit the discolorationof the paperboard when used in containers to package detergentscontaining persalts. More specifically, this invention pertains to animproved process based on the disclosure in U.S. Letters Patent No.3,536,578 assigned to the present assignee.

In the above-identified patent the use of persalts as bleaching agentsin detergent formulations was recognized. Moreover, it was pointed outin the same patent that persalts, and especially sodium perborate, havebeen getting increased attention as bleaching agents in detergentcompositions.

However, the use of sodium perborate in detergents has had certaindisadvantages. Because of its high oxidative efficiency, paper orpaperboard containers for the detergents become discolored duringstorage and are no longer esthetically appealing. The discolorationoccurs on both the inside and the outside surfaces of containers inwhich the sodium perborate containing detergents are packaged.Generally, the inside surfaces have spots of brown color and in the mostsevere cases, the inside surfaces assume a substantially uniform browncolor, whereas the outside surfaces of the containers may have a mottledappearance, or a non-uniform browning consisting of dark brown areasadjacent to light yellow areas.

The prior patent hypothesized that the discoloration of the paperboardWas due to the oxidative degradation of carbohydrate materials, andprotein materials present in the board. It was suggested that duringstorage, and, especially in hot and humid areas, the perborate materialin the detergent became active, thereby releasing hydrogen peroxidewhich oxidatively reacted with carbohydrate materials such as thecellulose in the paperboard and starch materials in and on thepaperboard.

In terms of the container in which the perborate containing detergent ispackaged, these reactions generally mean that the browning of thecontainer on the inside surface involves the discoloration of thecellulose of the paperboard and the starch in the sizing, and thediscoloration and mottling on the outside surface of the container isdue to the oxidation products of cellulose, starch in the sizing andcoating, and protein in the coating. The mottling on the outsidesurfaces of the containers is beleived to be caused by borate materialswhich migrate through the paperboard.

3,723,164 Patented Mar. 27, 1973 As noted in the prior patent however,it was found that the mottle or discoloration problem could beeliminated by manipulating the surface pH of the paperboard. This wasdone by treating the paperboard with chemical materials to provide thepaperboard with an alkaline surface pH within the range of from 8.0 to10.5, with a preferred range between about 9.0 to 10.0. Testsillustrated that the surface pH was the controlling factor and not thepH of the substrates beneath the surface layers which in most casescontinued to have an acid pH after application of the alkali salts tothe paperboard surface. The prior patent thus described and claimed aprocess for manufacturing paper and paperboard which included a sizingstep wherein the paperboard was treated with an aqueous solution of aninorganic alkali salt to provide the paperboard with a surface pH ofabout 8.0 to 10.5. Specific claims in the prior patent indicated sodiumnitrite to be preferred salt solution. Further studies of the mottlingphenomena of perborate containing detergent con tainers formed frompaperboard treated as described in the above-noted patent illustratedthat while sodium nitrite was an efiective weapon against the occurrenceof mottle, its protective ability was time-limited and dependent onhumidity and temperature. I

DETAILED DESCRIPTION As noted hereinbefore, it was found that perboratecontaining detergents caused the paperboard from which detergentcontainers were made to become mottled. The invention described in theprior U.S. Pat. No. 3,536,578 showed that the application of sodiumnitrite, a potent oxygen acceptor, to the surface of the paperboard in asurface sizing treatment to raise the surface pH to around -10, provedto be an effective supressant to the formation of mottle. Laterobservations indicated, however, that under extremely adverse climaticconditions typical of the Southern and Southeastern United States, thesodium nitrite became unstable after a few months storage. Detergentcontainers collected from supermarkets located in Miami, Fla., and,Houston, TeX., contained only a trace of nitrite after six monthsstorage; and these containers also became mottle prone. Therefore, itbecame clear that the ability of the sodium nitrite to protect thepaperboard surfaces from ultimately becoming mottled was limited to thetime during which the sodium nitrite remained in the board. Moreover,the disappearance of the nitrite from the board was surprising since itwas known that nitrite solutions are normally fairly stable in theneutral or alkaline region, that is, above a pH of 5-6.

It was suggested that an application of excessive sodium nitrite to thepaperboard would prolong the presence of nitrite in the paperboard.However, under the extremely adverse climatic conditions encountered, itwas discovered that the excess alkali tended to promote the degradationof starch and cellulose yielding color reversion in the board along withan acidification of the board which was accelerated in the presence ofheat and humidity. Moreover, it was also demonstrated with experimentsthat heating of the moistened paperboard containing sodium nitrite,without detergent, at 50 degrees C. for 45 hours, removed almost all ofthe nitrite content. Thus it was decided that the stability of thesodium nitrite in the paperboard was directly afiected by a decreasingof the pH of the paperboard due to an acidification of the paperboard inthe presence of increased moisture and temperature. In addition, it wasdiscovered that in the detergent containers, the perborate from thedetergent would penetrate the board and react with the sodium nitriteunder these adverse conditions of humidity and temperature. Thus itbecame important to develop some means whereby the stability of thesodium nitrite in the board could be improved to keep the paperboardsubstantially mottle free under the most adverse conditions.

It was obvious that increased doses of sodium nitrite to the board tomaintain a continuously high surface pH was not the single answer, so,the activity of the perborate containing detergent was studied.

It was found that perborate was unstable in the detergent itself, andits decomposition was shown to increase with increases in moisture andtemperature. Under normal conditions, at room temperature, perborate intypical detergents lose their activity very slowly with about half ofthe activity remaining at the end of several years storage. At 60degrees C., the time to lose half activity is reduced to 12 days, and at110 degrees C., to 42 minutes. The calculated activation energy ofperborate was determined to be around 30 KcaL/ mole.

Sodium nitrite on the other hand is a salt of nitrous acid. When sodiumnitrite is hydroylzed by water:

Nitrous acid is unstable when heated in a dilute solution and easilydecomposes:

Further, while a strong oxidizing agent will convert the nitrite tonitrate, sodium nitrite can also act as an oxidant:

N gas is oxidized by air to N0 which in turn, gives nitric acid withwater:

Nitrous acid can act as an oxygen scavenger, but it may also contributeto oxidation of the carbohydrates or other substances in the board.

Therefore, one of the probable roles of sodium nitrite is to react withthe perborate and deprive it of the obnoxious oxygen. Studies indicatedthat the rate of oxidation of sodium nitrite to sodium nitrate byperborate was quite rapid at first, but increased at a slower rate withincreasing temperature than, for instance, the rate of perboratedecomposition. The role of the sodium nitrite in the board is, ofcourse, to conteract the penetrating perborate solution and to reactwith the perborate before it gets a chance to oxidize the surfaces ofthe paperboard.

In addition, in dilute solutions, the stability of sodium nitrite wasfound to be strongly pH dependent. At 50 degrees C. and at a pH greaterthan 5.5, the sodium nitrite was stable, whereas at a pH of 4.5, therewas a loss after 50 hours and a loss after 100 hours. If perborate waspresent, the reaction product was determined to be nitrate. At 50degrees C. and at a pH greater than 5.5 in the presence of perborate,there was a 6 conversion of the sodium nitrate after 50 hours. With thesame solution at a pH of 4.5, and under identical temperatureconditions, there was an 85% conversion of sodium nitrite to sodiumnitrate after 50 hours. And, of course, the sodium nitrate is completelyineffective as a means for preventing mottle in paperboard.

On the other hand, when a solution containing between 02-05% sodiumnitrite with about 10% moisture was applied to paperboard in the absenceof perborate, and the board was placed in a closed container at 50degrees C. for 45 hours, at least 85 to 95% of the nitrite content waslost. This loss was due partly to an oxidation in the form of nitrateand partly in other ways. Hence, these experiments illustrated that eventhrough the pathway of nitrite degradation in the board was complicatedand not completely understood, certainly the propensity of the board tomottle could be arrested if some means were discovered for stabilizingthe sodium nitrite in the board. Further experiments showed that thenitrite could in fact be stabilized in the board by the application of abutfering agent. The preferred treatment was found to comprise the useof magnesium carbonate, but, both calcium carbonate and sodium. si icatewere alsQ found to gi sa i a t y results in retaining a part of thenitrite in the paperboard even after aging of the board under extremelyadverse conditions.

Therefore, for the present invention, a buffering agent was added to thepaperboard in addition to the sodium nitrite treatment, to stabilize thesodium nitrite in the paperboard with satisfactory results. Thepaperboard was made substantially as before, as taught in US. Pat. No.3,536,578, except that at the size press, where normally a solution ofclay and starch is applied, in two cases a part of the clay was replacedwith either calcium carbonate or magnesium carbonate, and in anothercase, sodium silicate was added to the size press mixture in addition tothe normal clay and starch solution.

As shown in Table I, the paperboard for the present invention was madein a conventional manner and then applied with a sizing treatment onboth sides of between 1 and 2 pounds per side. The sizing treatmentvaried, comprising a conventional sizing with and without sodiumnitrite, and with varying amounts of clay and different bufferingagents. After drying, the sized paperboard was then coated on one side(wire side) with a conventional coating composition comprising adhesiveand pigment. The paperboard was then used to construct detergentcontainers which were tested both empty and containing detergent.

It should be appreciated however, that the sodium nitrite treatmentcould be made at the waterbox on a calender after the sizing treatmentinstead of simultaneously with the sizing if desired. The prior US. Pat.No. 3,536,578 clearly illustrated that the place of application of thesodium nitrite to the paperboard was equally effective both at the sizepress or at the waterbox. In addition, the sodium silicate bufferingtreatment could also be successfully applied at the waterbox if desiredwith equally satisfactory results. Of course, when either calciumcarbonate or magnesium carbonate were used as the buffering agent, theapplication would have to be in the form of a size press treatment.

In the examples which follow, the size press mixtures each consisted ofcertain standard ingredients with other ingredients added as shown byTable I.

TABLE I Preparation of Size Press Mixtures Standard Ingredients:(percent solids 13-16) Starch PG 230: (450 gr.) Starch PG 260: (150 gr.)50% NaOH: (20 gr.) Carbowax 3000: (15 gr.) Calgon: (10 gr.)

DC A: (40 gr.)

Additional size press ingredients (grams) No. 2 Parez Ex. clay 613 CaCO3 MgCOa NazSlO; NaNOz Water 850 (Control) 11, 000 850 200 10, 500 425200 12, 000 425 425 200 11, 000 800 200 200 10, 000

As shown in Table I, the first example was the control and consistedonly of the standard size press ingredient plus clay and Parez 613 amelamine formaldehyde resin. The board in Example 2 was applied with astandard size press treatment including the addition of sodium nitriteas disclosed in the prior US. Pat. No. 3,536,578. The remaining threeexamples were treated at the size press equal parts of clay and calciumcarbonate (Example 3); equal parts of clay and magnesium carbonate(Example 4); and, with clay plus sodium silicate (Example 5 The examplesnoted above were then put through two tests, an accelerated aging testto determine the retention of the nitrite in the paperboard underadverse conditions, and, a simulated field test where after 4 weeks ofaging, visual observations were made a d recorded to show the TABLE IIAccelerated Aging Test 50 degrees 0., 46 hours (+10% moisture) InitialAfter aging NOz-N NOz-N Percent Examples (p.p.m.) (p.p.m.) retentionTABLE III Aging of Paperboard with Bold Detergent 50 degrees C.4 weeksInitial N O 2-N (p.p.m.) Visual observations No gottle felt side-Heavymottle outside.

Examples 2. 290 o. 3 255 No mottle felt side-Very light outside. 4- 390Do.

In another example using paperboard produced at Westvacos Covington, Va.paper mill, the paperboard was subjected to the same tests describedabove for the paperboard produced on a laboratory paper machine. In 3this latter example, the paperboard was made substantially as describedin prior US. Pat. No. 3,536,578 except that the sodium nitrite treatmentwas applied at a waterbox on the calender and calcium carbonate wassubstituted for one-half the clay at the size press. For this example,the percent retention of nitrite in the board after the acceleratedaging test at 50 degrees C. for 45 hours (with 10% moisture),illustrated that 28% of the nitrite remained in the board. In the visualtest for the determination of mottle after aging the paperboardcontainers packaged with Bold detergent at 50 degrees C. for 4 weeks,the container showed no mottle. Thus it can be shown by this examplethat it is not critical where the nitrite is applied, only that it iscritical whether or not the buffering agent is added.

In the accelerated aging test (Table II), the percent retention'ofnitrite in the paperboard was measured after the board underwent thefollowing test. Detergent containers were fabricated from the paperboardand a known amount of water (approximately 5-8 percent) was applied tothe surface of the paperboard to simulate the high humidity conditionswhich were found to be troublesome. The tightly closed containerswithout detergent were then placed in an oven at 50 degrees C. and leftfor 45 hours to accelerate the aging of the board before measuring thenitrite retained.

For the visual observations to determine the amount of mottle protectionafforded with the addition of buffering agents (Table III), both treatedand untreated samples of the paperboard were used to construct detergentcontainers, and the containers were then filled with detergent whichincluded sodium perborate as the bleaching agent. The coated or wireside of the paperboard was the outside surface of each detergentcontainer in each example. The filled containers were then grouped insets of two or three boxes and wrapped and sealed initially with kraftpaper. A second double layer wrap of aluminum foil was then applied andall seams sealed prior to placing the packages in the oven. The packageswere then placed in an oven at 50 degrees C. for 4 weeks, and afterwardsI the center-board surfaces of each container were checked visually formottle and with a reflectance instrument. The test was carefullydesigned to simulate as closely as possible typical containers on asupermarket shelf under the extremely harsh environmental conditionsencountered in the problem areas of the country. Of course, as shown inTable III, the treated boards performed quite satisfactorily. Inaddition, both the accelerated aging test of the board to determine theretention of the sodium nitrite in the paperboard, and the extendedaging test of the paperboard containers containing detergent have beenshown to correlate well with actual field results.

From the above data, it can be seen that when sodium nitrite is appliedeither with a size press treatment or a waterbox treatment topaperboard, it can be made to remain in the paperboard with a suitablebuffering agent which is also applied with the size press treatment.Moreover, the data also shows that when the nitrite is somehow fixed orretained in the paperboard by the buffering agent, a substantialreduction in the mottling tendency of the coated side of the paperboardcan be achieved.

While the theory behind the effectiveness of providing the paperboardwith the bufifering agents to fix the nitrite in the board is not fullyunderstood, it is believed that without the addition of a bufferingagent, the moist fiber surface accelerates the hydrolysis of nitritethus:

And the nitrous acid formed undergoes several types of reactions(oxidation, nitration, decomposition) with each of these reactionsresulting in the effective loss of the protective agent nitrite. Thebuifering agent tends to prevent this hydrolysis and thereby retains thenitrite in its substantially inert form. The buffering agent is not tooeifective however, and still releases just enough nitrous acid to reactwith the perborate when it penetrates the board under the extremelysevere conditions of temperature and humidity.

Since the protective action of the nitrite is based primarily on itsreaction with perborate, too much alkalinity would render the oxygenacceptability of the nitrite minimal. The compromise of course is tokeep the nitrite in its reactive state but still prevent it from beinghydrolyzed. Apparently the selected buffering agents serve this purposeFrom the above example, it is apparent that the treatment of paperboardwith sodium nitrite, as described in the prior US. Pat. No. 3,536,578,and, with a buffering agent as described herein, presents an effectivedeterrent to the discoloration and mottle problems under conditions ofhigh humidity and temperature, that were found in paperboard containersfor perborate containing detergents. Specifically, the treatmentdisclosed herein which involves only an addition of from 1025 pounds ofbuffering agent per ton of paper produced, satisfies the requirement ofmottle-free paperboard in extremely adverse climatic conditions.

Obviously, however, since only several specific treatments have beenfully disclosed herein, it is believed that other various changes may bemade in the treatments described without departing from the spirit ofthe invention or the scope of the appended claims.

We claim:

1. Paperboard, suitable for use in containers to package detergentscontaining a persalt such as sodium perborate as a bleaching agent,bearing on at least one side thereof a coating containing starch andsodium nitrite, the improvement comprising an insoluble buffering agentin the coating selected from the group consisting of magnesiumcarbonate, calcium carbonate or sodium silicate for the purpose ofstabilizing the sodium nitrite in the paperboard and preventing thediscoloration of the carbohydrate and protein materials in and on thepaperboard under adverse climatic conditions.

2. In the process of manufacturing paperboard which comprises the stepsof sizing the paperboard with a sizing treatment containing starch andsodium nitrite, coating at least one surface of the paperboard with acoating composition that includes pigment and binder, and then dryingthe paperboard, the improvement for the prevention of mottling of thepaperboard in the presence of perborate containing detergents,consisting of applying to the paperboard an insoluble buffering agentselected from the group consisting of magnesium carbonate, calciumcarbonate or sodium silicate for the purpose of stabilizing the sodiumnitrite in the paperboard under conditions of high humidity andtemperature.

3. The process of claim 2 wherein the sodium nitrite and the bufferingagent are applied to the paperboard simultaneously with the sizingtreatment.

4. The process of claim 2 wherein the sodium nitrite and the bufferingagent are applied to the paperboard in separate treatments.

References Cited UNITED STATES PATENTS Bowlby 11786 Yowtz 162160 Fleck.

Brundige 162206 Bolt 212.5

Baseman 212.5 Brundige et al. 117169 X WILLIAM D. MARTIN, PrimaryExaminer M. R. LUSIGNAN, Assistant Examiner US. Cl. X.R.

